This invention relates to a converter for exothermic reactions, and more particularly to a converter and method such as for converting nitrogen and hydrogen to ammonia whereby reduced catalyst usage and/or greater product yields are obtainable.
Ammonia is commonly manufactured by reacting nitrogen and hydrogen in a synthesis loop which can include a compressor, an ammonia synthesis reactor, and an ammonia condensation and recovery step. Unreacted synthesis gas from the synthesis reaction is typically recycled from the ammonia separator back to the compressor and reactor. The synthesis gas can contain argon, methane, and other inert components which are typically removed as a purge stream, thereby avoiding buildup of inerts in the synthesis loop. The purge gas can be further processed in a hydrogen recovery unit, or alternatively supplied directly to the fuel system with or without additional treatment or hydrogen recovery.
Many ammonia production plants operate with a synthesis loop using an iron-based magnetite catalyst in the ammonia converters. Significant advances in the manufacture of ammonia have included the use of highly active synthesis catalysts comprising a platinum group metal supported on graphite-containing carbon, used alone or in conjunction with less active iron based catalysts, as described in U.S. Pat. Nos. 4,568,530, 4,568,531, and 4,568,532. Desirably, the platinum group metal is ruthenium, as more fully described in U.S. Pat. Nos. 4,122,040 and 4,250,057. The highly active catalysts generally allow for increased ammonia production and/or the usage of smaller volumes of catalyst.
In general, contact of the reactants with a catalyst under suitable temperature and pressure conditions effects an exothermic reaction. The heating associated with exothermic reactions can have various positive and negative effects on the reaction. Negative effects can include: poor production rates, deactivation of catalyst, production of unwanted by-products, and damage to the reaction vessel and piping. Most commonly, an excessive temperature increase in the reaction zone either limits selectivity or reduces product yield.
Exothermic reaction processes can encompass a wide variety of feedstocks and products. Examples of moderately exothermic processes can include methanol synthesis, ammonia synthesis, and the conversion of methanol to olefins. Examples of highly exothermic reactions can include oxidation reactions in general, phthalic anhydride manufacture by naphthalene or orthoxylene oxidation, acrylonitrile production from propane or propylene, acrylic acid synthesis from acrolein, the conversion of n-butane to maleic anhydride, the production of acetic acid by methanol carbonylation, and methanol conversion to formaldehyde.
The efficiency of reversible exothermic reactions often depends on the ability to remove the heat generated by the process. The reaction rate and equilibrium generally move oppositely with increasing temperature. Thus, higher reaction temperatures generally result in faster reaction rates and lower overall conversion, while lower reaction temperatures generally result in slower reaction rates and higher overall conversion. For increased conversion in staged reversible exothermic reactions, a high temperature is employed in the early stages of the reaction where the reaction kinetics are more favorable. As the reaction progresses, the temperature in the later stages is reduced to take advantage of the more favorable equilibrium conditions. However, because the reaction is done in stages with interstage cooling, the equilibrium and kinetics are rarely, or only for very briefly, balanced for the maximum reaction rate possible. The present invention employs conditions approximating the optimal reactor operating curve (or temperature progression) which maximizes the reaction rate along a path corresponding to a locus of maximum rates on a temperature-conversion plot. This type of plot generally follows a decreasing temperature profile moving from the reactor inlet to outlet.
Some prior art reactors have relied upon arrangements that contain the reactions in generally adiabatic reactor zones and supply indirect contact with a cooling medium between stages. The geometry of intercooled reactors poses layout constraints that require large reactors and vast tube surfaces to achieve high heat transfer efficiencies. In U.S. Pat. No. 4,696,799, Noe discloses an ammonia synthesis converter having shell and tube interchangers for cooling the reaction gas streams leaving the catalyst beds with incoming reactant gases. In U.S. Pat. No. 5,869,011, Lee discloses a fixed bed reactor that partitions a single stage catalyst bed into multiple heat interchanged stages in a single reaction vessel.
In U.S. Pat. No. 6,171,570, Czuppon discloses maintaining a substantially isothermal condition by boiling water on the shell side of a shell-and-tube reactor with catalyst-filled tubes. Disclosed advantages include lower energy consumption, lower purge rates, and higher ammonia production rates. While overall catalyst efficiency can be better than is found in reactors operating at adiabatic conditions, of course, the isothermal condition means that towards the outlet of the reactor, the reaction composition may still approach equilibrium product concentrations, thus limiting further reaction. For an exothermic reaction such as ammonia production from hydrogen and nitrogen, the product ammonia concentration at the outlet end of an isothermal reactor can be higher than the product concentration at the outlet of the adiabatic reactor given sufficient reaction time. This is true because in the adiabatic reactor, with an exothermic reaction such as ammonia production from hydrogen and nitrogen, temperatures increase along the reactor length and the equilibrium product concentration of ammonia is lower at higher temperatures.
Adiabatic fixed bed reactors with interstage cooling have been used in the prior art to provide successive conversion at lower and lower temperatures to improve catalyst efficiency and improve yields. In practice, prior art reaction processes have been limited to two to four stages in one common reactor vessel, with the major limitation being the capital costs associated with interstage heat exchange equipment and multiple reactor stages and/or vessels. In addition, inlet temperature at each bed is necessarily lower than the outlet temperature, which is closer to the equilibrium temperature. For example, in U.S. Pat. No. 6,015,537, Gam discloses a reactor for the preparation of ammonia from a synthesis gas featuring multiple catalyst beds with intermediate cooling of partially converted synthesis gas between each catalyst individual bed.
In one commercially available prior art ammonia process, four catalysts beds are provided with inter-cooling between each of the beds. The first bed, and sometimes the second bed, can feature an iron-based magnetite catalyst, followed by two or three beds which contain a ruthenium-based catalyst. Reactor temperature at the inlet of each catalyst bed is low due to the increasing temperature profiles in the adiabatic exothermic ammonia synthesis reaction zones. The exothermic nature of the reaction, together with the adiabatic reactor bed design, do not allow the temperature profile to maximize per-pass ammonia conversion, in turn leading to inefficient catalyst use. In such a system, larger amounts of the catalyst are necessary to achieve higher per pass ammonia conversion.
Similarly, isothermal reactors have limitations in the production of ammonia. Synthesis of ammonia using an isothermal reactor generally requires separate external preheating of the feed gas. Additionally, as with the staged adiabatic reactors, typical isothermal reactors have relatively high catalyst requirements to obtain equivalent conversion rates.
Accordingly, there is a need in the art for a reactor design which controls the temperature of exothermic reactions along the length of the reactor that effectively utilizes a temperature:conversion operating curve that follows the equilibrium curve with a negative temperature offset, and thus maintains a high reaction rate and catalyst efficiency throughout the catalyst bed volume.